In a liquid ring expander, an impeller with blades mounted on it is mounted eccentrically in an expander body. A service liquid is present in the expander body and is flung against the wall of the expander body as a result of the centrifugal forces generated by rotation of the impeller. The volume of the service liquid is less than the volume of the expander body. In this way, the service liquid in the expander body forms a circumferential liquid ring which forms chambers bounded in each case by two blades and the liquid ring. Owing to the eccentric positioning of the impeller in the expander body, the size of the chambers increases in the direction of rotation of the impeller, thus allowing gas introduced at high pressure into the narrow chambers of the expander to expand and thereby rotate the impeller.
A liquid ring compressor operates in an analogous manner, only in this case gas is introduced into the widest chamber of the expander such that the size of the chambers decreases in the direction of rotation of the impeller. Owing to the rotation of the impeller and the reduction in the size of the chambers, the gas which has been drawn in is compressed and ejected from the liquid ring expander on the high pressure side.
US 2008/0314041 (corresponding to IL 163263) in the name of the present inventor discloses a heat engine that includes at least one Liquid Ring Rotating Casing Compressor (LRRCC) having a fluid inlet and a fluid outlet, a combustion chamber in fluid communication with the output of the LRRCC, and at least one expander having a fluid inlet and a fluid outlet. The fluid inlet communicates with the combustion chamber. Efficient LRRCC compressors/turbines are also known from EP 804 687.
The contents of both US 2008/0314041 and EP 804 687 are incorporated herein by reference.
In the heat engine described in US 2008/0314041, an LRRCC is used in tandem with an expander, which may be a conventional turbine or a liquid ring expander of the kind described above. In the case where the turbine is a liquid ring expander having a rotating casing, air at high pressure and high temperature is injected into the casing so as to rotate the impeller.
Liquid ring turbines are only feasible if the casing rotates together with the impeller since the friction between the impeller and a fixed casing is prohibitive to obtaining reasonable efficiency. Rotating casing rotating liquid ring turbines are known in the literature but have so far been only theoretical based on the physical principle that an expander is complementary to a compressor. While this is, of course, true in principle, practical rotating casing liquid ring turbines do not appear to have been realized and most turbines currently in use employ very high pressure steam to rotate the turbine at high speeds. As is well-known, several turbines are often employed in cascade, the steam emitted from one turbine being use to rotate the next turbine and so on, until the pressure of the steam is too low to be of effective use. The steam is then cooled using cold water which may come from a river, the sea or a cooling tower.
The use of steam in a rotating casing rotating liquid ring turbine has been proposed by U.S. Pat. No. 4,112,688 (Shaw), which describes a rotating liquid ring turbine driven by an expanding gas and having a rotating casing. Shaw requires that no change of phase occurs in the energy transfer medium as, for example, occurs in the case of the Rankine turbine cycle in which water is converted to steam and back again with unavoidable energy losses, and reduced operating efficiency.
However, in order to meet this requirement, energy must be constantly supplied during the expansion phase to maintain the working medium as steam and thus prevent it from condensing. This is achieved by the provision of heat exchangers in the impeller.
As described, for example, in Wikipedia®, use of the Rankine cycle is well established in steam turbines where a pump is used to pressurize working fluid received from a condenser as a liquid instead of as a gas. All of the energy in pumping the working fluid through the complete cycle is lost, as is all of the energy of vaporization of the working fluid, in the boiler. This energy is lost to the cycle in that first, no condensation takes place in the turbine; all of the vaporization energy being rejected from the cycle through the condenser. But pumping the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it as compared to compressing the working fluid as a gas in a compressor (as in the Carnot cycle).
The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapor with entrained droplets often seen billowing from power stations is generated by the cooling systems (not from the closed-loop Rankine power cycle) and represents the waste energy heat (pumping and vaporization) that could not be converted to useful work in the turbine.
One of the principal advantages the Rankine cycle holds over others is that during the compression stage relatively little work is required to drive the pump, the working fluid being in its liquid phase at this point. By condensing the fluid, the work required by the pump consumes only 1% to 3% of the turbine power and contributes to a much higher efficiency for a real cycle. The benefit of this is lost somewhat due to the lower heat addition temperature as compared with gas turbines, for instance, which have turbine entry temperatures approaching 1500° C. FIG. 1 is a Temperature (T)-Entropy (S) diagram for the conventional Rankine cycle (based on open source data in Wikipedia®), showing that there are four processes identified as follows:
Process 1-2: The working fluid is pumped from low to high pressure; as the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.
Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.
Process 4-1: The wet vapor then enters a condenser external to the turbine where it is condensed at a constant pressure to become a saturated liquid.
In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown in FIG. 1 prevents the vapor ending up in the superheat region after the expansion in the turbine, which reduces the energy removed by the condensers.
Point 3 lies on the envelope of the T-S curve that delineates between vapor and gas. Thus, if the working fluid is water, to the right of point 3, the working fluid is pure steam while to the left, i.e. within the envelope of the T-S curve it is wet steam and to the left of point 1, it is water. In practice, it is considered undesirable in a practical turbine to reduce the temperature of the working fluid from 3 to 4 since the steam is wet and when water droplets impinge at high pressure on the turbine blades they are liable to cause damage such as pitting and erosion of the blades. This derogates from the performance of the turbine and in time causes irreversible damage, rendering the blades unusable. This problem has been solved using special materials that are resistant to erosion, but these are very expensive.
To avoid pitting caused by wet steam while using conventional materials, it is common to employ superheating of the steam at point 3, so as to raise the temperature to close to 1,000° C. before being directed on to the turbine blades. Superheating, shown by the chain-dotted line, dries the steam thus avoiding the problem of pitting of the turbine blades. Typically, the steam is allowed to condense to a point denoted by 5 on the T-S curve, where its temperature is much reduced and is then re-heated and directed again on to the turbine blades as dry steam where it loses heat and strikes the T-S curve at point 6 where its entropy (S) is significantly higher than that for the conventional Rankine cycle without superheating.
In summary, the Rankine cycle requires either that special materials are used for the turbine blades in which case isentropic heat-energy conversion is possible but at the cost of highly expensive turbine blades; or superheating is required so as to ensure that during the heat-energy conversion stage the steam is maintained dry. This reduces the overall efficiency of the engine.
The present invention seeks to offer the benefits of a near-Rankine cycle which is essentially isentropic without requiring the steam to be dry during the heat-energy conversion stage.